Return byte string for undefined BGP generic transitive extended communities sub-types

[tonusoo]

Scapy currently fails to decode a BGP UPDATE message if it contains a generic transitive extended community with an undefined sub-type. Example:

>>> BGPHeader(bytes.fromhex("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF0068020000004D4001010040020A020200000B62000056374003045051C02E80040400008A60C008100B62019A0B62057A0B6209630B620D48C010180005000056370200\
80000000000005008001C71B5402000018C71B54"))
<BGPHeader  marker=0xffffffffffffffffffffffffffffffff len=104 type=UPDATE |<Raw  load=b'\x00\x00\x00M@\x01\x01\x00@\x02\n\x02\x02\x00\x00\x0bb\x00\x00V7@\x03\x04PQ\xc0.\x80\x04\x04\x00\x00\x8a`\xc0\x08\x10\x0bb\x01\x9a\x0bb\x05z\x0bb\tc\x0bb\rH\xc0\x10\x18\x00\x05\x00\x00V7\x02\x00\x80\x00\x00\x00\x00\x00\x05\x00\x80\x01\xc7\x1bT\x02\x00\x00\x18\xc7\x1bT' |>>
>>>

Hex string above represents a BGP UPDATE message containing two generic transitive extended communities (type 0x80) and both have a sub-type which does not get handled in m2i() method of _ExtCommValuePacketField() class.

With this patch, Scapy successfully decodes the same BGP UPDATE message and returns a byte string for the value of the extended community:

>>> BGPHeader(bytes.fromhex("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF0068020000004D4001010040020A020200000B62000056374003045051C02E80040400008A60C008100B62019A0B62057A0B6209630B620D48C010180005000056370200\
80000000000005008001C71B5402000018C71B54")).payload.path_attr[-1].show()
###[ BGPPathAttr ]###
  type_flags= Transitive+Optional
  type_code = EXTENDED COMMUNITIES
  attr_len  = 24
  \attribute \
   |###[ EXTENDED_COMMUNITIES ]###
   |  \extended_communities\
   |   |###[ EXTENDED_COMMUNITY ]###
   |   |  type_high = Transitive Two-Octet AS-Specific Extended Community
   |   |  type_low  = OSPF Domain Identifier
   |   |  \value     \
   |   |   |###[ Two-Octet AS Specific Extended Community ]###
   |   |   |  global_administrator= 0
   |   |   |  local_administrator= 1446445568
   |   |###[ EXTENDED_COMMUNITY ]###
   |   |  type_high = Generic Transitive Experimental Use Extended Community
   |   |  type_low  = OSPF Route Type (deprecated)
   |   |  \value     \
   |   |   |###[ Raw ]###
   |   |   |  load      = b'\x00\x00\x00\x00\x05\x00'
   |   |###[ EXTENDED_COMMUNITY ]###
   |   |  type_high = Generic Transitive Experimental Use Extended Community
   |   |  type_low  = OSPF Router ID (deprecated)
   |   |  \value     \
   |   |   |###[ Raw ]###
   |   |   |  load      = b'\xc7\x1bT\x02\x00\x00'

>>>

This leaves it up to the application using the Scapy library to further process those values, if needed.

[gpotter2]

Thanks for the PR.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.20%. Comparing base (57db4aa) to head (4de8cc2).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/contrib/bgp.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4745      +/-   ##
==========================================
- Coverage   82.20%   82.20%   -0.01%     
==========================================
  Files         361      361              
  Lines       87195    87196       +1     
==========================================
- Hits        71680    71679       -1     
- Misses      15515    15517       +2     

Files with missing lines	Coverage Δ
scapy/contrib/bgp.py	91.92% <0.00%> (-0.13%)	⬇️

... and 6 files with indirect coverage changes

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